Scintillators are materials that convert high-energy radiation, such as X-rays and gamma rays, into visible light. Scintillators are widely used in detection and non-invasive imaging technologies, such as imaging systems for medical and screening applications. In such systems, high-energy photons (e.g., X-rays from a radiation source) typically pass through the person or object undergoing imaging and, on the other side of the imaging volume, impact a scintillator associated with a light detection apparatus. The scintillator typically generates optical photons in response to high-energy photon collisions. The optical photons may then be measured and quantified by light detection apparatuses, thereby providing a surrogate measure of the amount and location of high-energy radiation incident on the light detector (usually a photodetector).
For example, a scintillator panel is typically used in computed tomography (CT) imaging systems. In CT systems, an X-ray source emits a fan-shaped beam towards a subject or object capable of being imaged, such as a patient or a piece of luggage. The high-energy photons from X-rays, after being attenuated by the subject or object, collide with a scintillator panel. The scintillator panel converts the X-rays to light energy (“optical photons”) and the scintillator panel illuminates, discharging optical photons that are captured by a photodetector (usually a photodiode) which generates a corresponding electrical signal in response to the discharged optical photons. The photodiode outputs are then transmitted to a data processing system for image reconstruction. The images reconstructed based upon the photodiode output signals provide a projection of the subject or object similar to those available through conventional photographic film techniques.
Resolution is a critical criterion for any imaging system or device, especially in CT systems and the like. In the case of CT and other like imaging systems, a number of factors can determine resolution; however, this application focuses on the scintillation panel and its effects on resolution. When a continuous, homogeneous scintillation layer is used, lateral propagation of scintillation light is known to reduce image resolution. For example, when optical photons are generated in response to X-ray exposure, these optical photons can spread out or be scattered in the scintillation panel, due to optical properties of the panel, and can be detected by more than one photodetector coupled to the scintillation panel. Detection by more than one photodetector usually results in reduced image resolution. Several approaches have been developed to help offset optical photon diffusion, including reducing the thickness of the scintillation layer. This reduces the distance the optical photons may travel in the scintillation layer. However, the thinner the scintillation layer, the lower the conversion efficiency since there is less scintillating material for a source radiation photon to stimulate. Thus, the optimum scintillation layer thickness for a given application is a reflection of the balance between imaging speed and desired image sharpness.
Another approach known in the art is to employ thallium doped cesium iodide (CsI:Tl) scintillation layers. Thallium doped cesium iodide scintillation panels have the potential to provide excellent spatial resolution for radiographic applications since CsI-based panels are able to display high X-ray absorptivity and high conversion efficiency. However, this potential is difficult to realize in practical applications due to the mechanical and environmental fragility of CsI-based materials. For example, CsI is highly water soluble and hygroscopic. Any scintillation panels made with CsI:Tl must be maintained in a sealed, low humidity environment to avoid attracting water that can negatively affect luminescence. CsI:Tl structures are also mechanically fragile, requiring special handling procedures during and after manufacture such as complete enclosure in shock resistant containers. As a result, production (and end product) costs are quite high in applications that have successfully realized the image quality benefit of thallium doped cesium iodide scintillation panels.
An alternative approach to using thallium doped cesium iodide is to increase the transparency of the scintillation layer in the scintillator panel. It is generally understood that a perfectly transparent scintillator panel would provide the highest spatial resolution. However, while the most transparent scintillator would be a single crystal, single crystal scintillator panels have not yet been constructed with practically useful dimensions and sufficient X-ray absorptivity for radiographic applications. Another option for increasing transparency is to disperse particulate scintillators in a polymeric matrix having a refractive index identical or closely similar to that of the scintillator; however, this approach requires a high loading of scintillator particles in the polymeric matrix, which to date has not yet been successfully achieved with practically useful dimensions and sufficiently high scintillator particulate loads.
While prior techniques may have achieved certain degrees of success in their particular applications, there is a need to provide, in a cost-friendly manner, transparent scintillator panels having not only image quality approaching that of CsI-based scintillator panels but also excellent mechanical and environmental robustness.